DESIGN IMPROVEMENT OF OUTER-ROTOR HYBRID EXCITATION FLUX

SWITCHING MOTOR FOR IN-WHEEL DRIVE ELECTRIC VEHICLE

WAN NORHASHIMAH BINTI WAN HUSIN

A project report submitted in partial

fulfillment of the requirement for the award of the

Degree of Master of Electrical Engineering

Faculty of Electrical and Electronics Engineering

Universiti Tun Hussein Onn Malaysia

JULY 2014

v

ABSTRACT

Due to in-wheel motors definite benefits of great controllability for each self-reliant

wheel as well as the convenience of more space of cabin due to conventional mechanical

transmission and differential gears are removed, more study and research of in-wheel

motors used in pure electric vehicles (EVs) propulsion systems have attracted and

involved great attention lately. Furthermore, more series batteries can be mounted to

gain the distance of driving. The main necessities are to have high torque density and

efficiency, since the motors are installed directly to the wheel. Because of high torque

possibility is required; a design of outer-rotor hybrid excitation flux switching motor for

in-wheel drive electric vehicle is suggested in this project. The suggested motor consists

of twelve (12) slots of stator poles, and ten (10) rotor poles. All these active parts are

placed on the stator. Secondarily, it has a steady rotor assembly which only contains a

single piece of rotor and has a wide range flux control abilities. Under some design

restrictions and specifications for the target electric vehicle drive applications, the

performance of the suggested machine on the initial design and improved design are

analyzed based on 2-D finite element analysis (FEA). The performance of the improved

design motor shows that the maximum torque achieved is 241.7921 Nm which is 72.61

% of the target performance, whereas the maximum power has achieved 143.47 kW

which is greater than the target value. Therefore, by extra design optimization it is

estimated that the motor will successfully reach the target performance.

vi

ABSTRAK

Oleh kerana motor dipasang terus ke roda, pastinya kebolehkawalan bagi setiap roda

adalah yang terbaik serta mempunyai ruang kabin lebih luas disebabkan sistem enjin

dan gear mekanikal yang biasa ditanggalkan, maka lebih banyak kajian dan

penyelidikan berkenaan pemasangan motor terus ke roda yang digunakan bagi kereta

elektrik mendapat perhatian ramai pengkaji. Tambahan pula, bateri siri boleh dipasang

bagi menambah jarak pemanduan. Keperluan utama adalah untuk mendapatkan

ketumpatan daya kilas dan kecekapan yang tinggi apabila motor dipasang terus ke roda.

Oleh kerana daya kilas yang tinggi diperlukan, rekabentuk outer-rotor hybrid excitation

flux switching motor adalah dicadangkan dalam projek ini. Cadangan motor terdiri dari

12 slot kutub pemegun dan 10 kutub pemutar. Bahagian aktif seperti magnet, angker

berlilit dan gegelung pengujaan berada di pemegun. Sebagai tambahan, ia mempunyai

satu binaan pemutar yang stabil di mana mempunyai pelbagai kebolehan kawalan fluks.

Di bawah beberapa sekatan dan spesifikasi reka bentuk bagi sasaran aplikasi kereta

elektrik, prestasi motor yang dicadangkan bagi rekaan awal dan rekaan penambahbaikan

dianalisa berdasarkan kepada 2-D finite element analysis (FEA). Prestasi bagi

rekabentuk penambaikan menunjukkan bahawa daya kilas mencapai nilai maksimum

pada 241.7921 Nm di mana ianya adalah 72.61 % bagi prestasi sasaran. Manakala

kuasa maksimum yang berjaya dicapai adalah 143.47 kW di mana nilai tersebut adalah

melebihi dari nilai sasaran. Oleh itu, dengan mengoptimumkan reka bentuk dianggarkan

motor akan berjaya mencapai prestasi sasaran yang dikehendaki.

vii

CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS AND ABBREVIATIONS xiii

CHAPTER 1 INTRODUCTION 1

1.1 Project Overview 1

1.2 Problem Statement 2

1.3 Objectives of Project 4

1.4 Scope of Project 4

CHAPTER 2 LITERATURE REVIEW 6

2.1 Introduction to Electric Motor 6

2.2 Review on Electric Vehicle (EV) 7

2.3 Review on Electric Motor used in Electric

Vehicle (EV)

8

2.4 Reviews on Flux Switching Motors (FSMs) 9

2.4.1 Permanent Magnet Flux Switching

Machine (PMFSM)

10

2.4.2 Field Excitation Flux Switching

Synchronous Machine (FEFSM)

11

viii

2.4.3 Hybrid Excitation Flux Switching Motor

(HEFSM)

13

2.4.4 Principle Operation of ORHEFSM 14

2.5 Relationship between Torque, Speed and Power 16

2.6 The Electric Motor Design Software 18

CHAPTER 3 METHODOLOGY 20

3.1 JMAG-Designer 20

3.2 Project Implementation in Geometry Editor 21

3.2.1 Rotor 22

3.2.2 Stator 23

3.2.3 Permanent Magnet (PM) 24

3.2.4 Field Excitation Coil (FEC) 25

3.2.5 Armature Coil (AC) 25

3.3 Project Implementation in JMAG-Designer 26

3.3.1 Material Setting 26

3.3.2 Conditions Setting 27

3.3.3 Circuit Setting 28

3.3.4 Mesh 29

3.3.5 Magnetic Study and Properties Setting 30

3.3.6 Simulation and Results Observation 31

3.4 The Design of 12Slots – 10Poles ORHEFSM 32

3.5 The Improved Design of 12Slots – 10Poles

ORHEFSM

34

CHAPTER 4 RESULT AND ANALYSIS 36

4.1 Introduction 36

4.2 Open Circuit Analysis 38

4.2.1 Armature Coil Arrangement Test 38

4.2.2 Zero Rotor Position Analysis 41

4.2.3 Flux Strengthening 42

4.2.4 Flux Lines and Flux Distributions 43

4.3 Load Analysis 45

ix

4.3.1 Torque versus Ja at various Je 45

4.3.2 Torque versus Speed 47

4.4 Motor Weight Calculation 48

4.5 Conclusion 48

CHAPTER 5 CONCLUSION AND RECOMMENDATION 49

5.1 Introduction 49

5.2 Conclusion 49

5.3 Recommendation 50

REFERENCES 51

x

LIST OF TABLES

1.1 ORHEFSM design restrictions and specification for

electric vehicle

5

2.1 Advantages and disadvantages of FSM 10

3.1 Description of icon used in Geometry Editor 22

3.2 Materials setting 27

3.3 Parameters of 12Slots - 10Poles ORHEFSM 33

3.4 The parameters of improved design 35

4.1 The differences of parameters between initial design

and improved design for the proposed ORHEFSM

37

4.2 Motor weight 48

xi

LIST OF FIGURES

1.1 Single motor drive for EV drivetrain system

configuration

3

1.2 In-wheel motor drive for EV drivetrain

configuration

3

2.1 Classification of electric motor 6

2.2 US light vehicle sales and fleet composition under

baseline scenario

8

2.3 Principle operation of PMFSM 11

2.4 Principle operation of FEFSM (a) θe = 0° and (b) θe

= 180° flux moves from stator to rotor (c) θe = 0°

and (d) θe = 180° flux moves from rotor to stator

12

2.5 The operating principle of the proposed HEFSM (a)

θe=0° - more excitation (b) θe=180° - more

excitation (c) θe=0° - less excitation (d) θe=180° -

less excitation

14

2.6 Principle operation of ORHEFSM (a) θe = 0º (b) θe

= 180º more excitation, (c) θe = 0º (b) θe = 180º less

excitation

16

2.7 Performance Characteristics of Electric Motor 17

3.1 Work flow of project implementation 21

3.2 Toolbar icons 22

3.3 Radial duplication of rotor 23

3.4 Radial duplication of stator 24

3.5 PM filled in the stator 24

3.6 FEC parts design 25

xii

3.7 Complete design of 12S-10P ORHEFSM 26

3.8 Step to material setting 27

3.9 Conditions setting 28

3.10 Circuit implementation (a) Armature coil circuit (b)

FEC circuit (c) UVW circuit

29

3.11 Mesh generated in ORHEFSM (a) Mesh properties

(b) Mesh view

30

3.12 Setting in study magnetic (a) Step control (b)

Conversion

31

3.13 Steps involved in simulate and results observation

(a) Run active case (b) Show data value

32

3.14 The design structure of ORHEFSM 32

3.15 Design parameters defined as D1 – D10 33

3.16 The improved design structure 35

4.1 Improved design structure of 12slot-10pole

ORHEFSM

37

4.2 Armature coil setting for coil test analysis 40

4.3 Initial three phase flux linkage 40

4.4 Improved three phase flux linkage 41

4.5 Zero rotor position for initial 12Slot – 10Pole

ORHEFSM 42

4.6 Zero rotor position for improved 12Slot – 10Pole

ORHEFSM 42

4.7 The ORHEFSM flux strengthening graph for both

designs 43

4.8 Flux lines 44

4.9 Flux distributions 44

4.10 Torque versus Ja at various Je of initial design 45

4.11 Torque versus Ja at various Je of improved design 46

4.12 Torque versus Ja at maximum Ja 46

4.13 Torque versus speed of initial design 47

4.14 Torque versus speed of improved design 47

xiii

LIST OF SYMBOLS AND ABBREVIATIONS

A

AC

DC

f

fe

fm

Ja

Je

k

kg

kW

mm

Nm

Nr

Ns

q

rms

rpm

r/min

s

T

V

Wb

Ѳ

⁰

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Ampere

Alternating current

Direct current

Frequency

The electrical frequency

The mechanical frequency

Maximum current density in armature coil

Maximum current density in FEC

Integer

Kilogram

Kilo Watt

Milimeter

Newton meter

The number of rotor poles

The number of stator slot

Number of phase

Root mean square

Revolutions per minute

Revolutions per minute

Second

Torque

Voltage

Weber

Angle

Degree

xiv

Ф

%

-

-

Flux

Percent

CHAPTER 1

INTRODUCTION

1.1 Project Overview

Demands toward vehicles for personal transportation have increased with world

population increasing. According to Kavan Mukhtyar, the Asia-Pacific partner and head

of Automotive and Transportation Practice, the average global population use private

transport was 53 percent [1]. The sheer number of vehicles on the road contributes to

global warming. These vehicles emit carbon dioxide which pollutes the atmosphere [2].

It is rather well-known that a new scientific finding that, counter to what most of us

believe, driving a car causes more global warming pollution than flying the same

distance in a plane [3]. The conventional internal combustion engine (ICE) vehicles are

the main contributors to this problem. This scenario occurs in many countries are no

exception to Malaysia. One of the solutions to the fuel consumption like petrol used to

propel the car would now focus on electrical technology. Because of environmental

problems likes the greenhouse effects are directly related to vehicles emissions, electric

vehicles have involved increasing interest from vehicle manufactures, governments and

consumers. The battery-powered electric vehicles (BEVs) seem like the best solution to

deal with energy and environmental problem since they zero oil consumption and zero

emissions [4]. The Malaysia Government through The Ministry of Energy, Green

Technology and Water has also looked towards the support of electric vehicles (EV) and

energy efficient vehicles (EEV) in the country offers the potential for Malaysia to

reduce need on fossil fuels. Therefore, to make possible the use of EV in the country,

the Government has finished the EV Infrastructure Roadmap, and the next phase is the

2

ongoing fleet test programmed for electric vehicles in Putrajaya and Cyberjaya. The

implementation of this fleet test will be the benchmark in developing a strategic plan

and framework as well as the identification of entities that will benefit the electric

vehicle industry in areas of services and new business opportunities [5]. Meanwhile,

Mukhtyar said, a research shows that 50 percent of transport consumer in Kuala Lumpur

would rather switch to fuel-efficient vehicles leading to an increase in eco-friendly

vehicles in Malaysia. On 2013, hybrid car sales are projected to grow by 4.2 percent to

16,000 units refer to the tax exemption and the launch of new models [1]. Electric car

giving many benefits to people so electric car was the best solution. Imagine a car

without gasoline, we do not need to go to the gas station to get petrol anymore but only

enough to charge in front of the house or parking of vehicles is provided. Presently,

research and development have focused on developing the electric vehicle. So, the

necessary conditions of basic characteristics of an electric motor for electric vehicle

drive system are: (1) high power and torque density; (2) wide speed range and constant

power; (3) high starting torque for hill climbing ability and high power in speed

cruising; (4) high efficiency over wide speed and torque ranges; (5) high reliability and

robustness appropriate to environment; (6) Intermittent overload ability and acceptable

cost; (7) Low acoustic noise and low torque ripple; (8) good voltage regulation over

wide speed [6].

1.2 Problem Statement

Nowadays the EV is become more popular. The common system configuration

applied in EV drivetrain is single motor drive which has centralized motor drive with

reduction gears and a differential axle as illustrated in Figure 1.1. The centralized drive

appears to be popular partly due to its similarity with existing ICE based system.

However this system has some shortcomings due to conventional mechanical

transmission and differential gear. The solution of in-wheel motor drive applied as

illustrated in Figure 1.2 has great controllability for each independent wheel as well as

the availability of more cabin space due to taking away of conventional mechanical

transmission and differential gears. In addition, more series batteries can be located to

3

increase the driving distance and long-lasting even though everyday use. Due to the

removal of mechanical transmission, differential gears and drive belts, the in-wheel

direct drivetrain provides fast torque response, higher efficiency, weight decrease and

increased vehicle space. At the same time, the cost to develop can reduce and the car is

able to be more compact.

Motor

Power

Source

Controller

Clutch + Gear

system +

Mechanical

Differential

Power

Source Controller Wheel

Motors

Inverter

Inverter

Figure 1.1: Single motor drive for EV drivetrain system configuration

Figure 1.2: In-wheel motor drive for EV drivetrain configuration

4

1.3 Objectives of Project

The objectives of this project are:

i. To design an Outer-Rotor Hybrid Excitation Flux Switching Motor for In-Wheel

Drive Electric Vehicles (ORHEFSM).

ii. To analyse the performances of the design under similar motor restrictions and

specifications installed in existing electric vehicle.

iii. To improve the torque and power performances of the design.

1.4 Scope of Project

The scope of project has been determined in order to achieve the objectives of this

project. The proposed motor consists of 12 slots of stator poles and 10 rotor poles. The

development of the project design is using JMAG-Designer 12.1.02 and the

implementation of the simulation and analyzed based on 2-D finite element analysis

(FEA). The design restrictions and specifications for the proposed motor ORHEFSM are

similar with interior permanent magnet synchronous motor (IPMSM) employed in

LEXUS RX400h and list in Table 1.1.

The target maximum torque and power are set to be more than 333 Nm and 123

kW. The target weight of the proposed motor is set to be at least similar as inner rotor

HEFSM which is 35 kg. Therefore, the proposed motor is expected to achive the

maximum power and torque density 0f 9.5 Nm/kg and 3.5 kW/kg, respectively. The

corresponding electrical restrictions to the inverter such as maximum DC bus voltage

and maximum inverter current are set to 650 V and 360 Arms. While, the maximum

current density in armature coil and field excitation field is set to 30 A/mm2 [7].

5

Table 1.1: ORHEFSM design restrictions and specification for electric vehicle

Descriptions Inner rotor

HEFSM

Outer-rotor

HEFSM

Max DC-bus voltage inverter (V) 650 650

Max. Inverter current (Arms) 360 360

Max. Current density in armature coil, Ja

(Arms/mm2)

30 30

Max. Current density in FEC, Je (A/mm2) 30 30

Motor radius (mm) 132 132

Motor stack length (mm) 70 70

Shaft/Inner motor radius (mm) 30 30

Air gap length (mm) 0.8 0.8

PM weight (kg) 1.1 1.1

Maximum torque (Nm) 333 >333

Maximum power (kW) 123 >123

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction to Electric Motor

Figure 2 1: Classification of electric motor

The electrical motor is a device that has brought about one of the biggest

advancements in the fields of engineering and technology ever since the invention of

electricity. An electric motor is an electro-mechanical device that converts electrical

energy to mechanical energy. There are different types of motor have been developed

for different specific purposes. The types of electric motor can be classified as shown in

Figure 2.1 above. Basically, electric motor divided into two groups which are direct

Electric Motor

Direct Current (DC)

Alternating Current (AC)

Induction Motor (IM)

Synchronous Motor (SM)

Permanent Magnet (PM)

Field Excitation

(FE)

Hybrid Excitation

(HE)

Switch Reluctance Motor (SRM)

Flux Switching Machine (FSM)

PMFSM FEFSM HEFSM

7

current and alternating current. Have four types in alternating current motor group

whereas induction motor (IM), synchronous motor (SM), switch reluctance motor

(SRM) and flux switching machine. Synchronous motor divided into three types, there

are permanent magnet, field excitation and hybrid excitation. Furthermore, flux

switching machine also divided into three as same as in synchronous motor.

All of these motors work in more or less same principle. An electromagnet is the

basis of an electric motor. An electric motor is all about magnets and magnetism.

Electric motor generally working depends upon the interaction between an electric

motor’s magnetic field and electric current to generate force within the motor.

2.2 Review on Electric Vehicle (EV)

All-electric vehicles (EVs) depend on electricity only. They are run by an electric motor

(or motors) powered by rechargeable battery packs. EVs have several advantages

compared to vehicles with internal combustion engines (ICEs). It is well known that

electric vehicles have the qualities of energy-saving, non-emission and low noise

pollution, and called as “green car”. A green car produces less harmful effects to the

environment than comparable conventional internal combustion engine vehicles running

on gasoline.

From a study of the Centre for Entrepreneurship and Technology at University

of California, Berkeley, electric cars could comprise 64-86% of US light vehicle sales

by 2030 [8]. The study in Figure 2.2 shows rapid acceptance for electric vehicles with

switchable batteries, calculates how the electrification of the United State (U. S.)

transportation system will cut America's necessity on foreign oil, increase employment,

and reduce the environmental effect of transportation emissions by conventional car.

From this study, the electric vehicle and the development of high performance electric

motor is a force to be reckoned with.

8

Figure 2.2: US Light Vehicle Sales and Fleet Composition under Baseline Scenario

2.3 Review on Electric Motor used in Electric Vehicle (EV)

An electric vehicle (EV) uses one or more electric motors for propulsion. Electric

motors give electric cars instant torque, creating strong and smooth acceleration using

electrical energy stored in batteries or another energy storage device. The electric motor

is the heart of electric vehicle which offer the driving part that moves electric vehicle in

many situation.

The electric motor comes in different shapes, driving methods, types and

function. Nevertheless, the necessary conditions of basic characteristics of an electric

motor for electric vehicle drive system need to studies as mentioned in past literature,

are summarized as follows:-

i. high power and torque density

ii. wide speed range and constant power

iii. high starting torque for hill climbing ability and high power in speed cruising

iv. high efficiency over wide speed and torque ranges

v. high reliability and robustness appropriate to environment

vi. intermittent overload ability and acceptable cost

vii. low acoustic noise and low torque ripple

viii. good voltage regulation over wide speed [6].

9

Many types of electric motor already used in study of research and

developments. Likes permanent magnet synchronous machines (PMSMs) with an outer-

rotor have been used as in-wheel direct drive motors for EVs, due to their high torque

density, superb efficiency and overload capability [9]. But, in extreme driving

conditions there probably will be risks of demagnetization and mechanical harm of the

rotor’s magnet. Alternatively, the switched reluctance motor (SRM) has the simplest and

strong rotor structure with no magnets, which makes it mainly tough against mechanical

and thermal effects. SRMs also have huge torque ripples and its make them

incompatible for direct drive. Lately, study and development on permanent magnet flux

switching machine (PMFSM) has turn into more interesting suitable to a number of

excellent characteristics of physical compactness, steady rotor construction, greater

torque, power density and great efficiency. So, the PMFSM inherits preferred

advantages of both of the PMSM and SRM [10-13]. Then, the outer-rotor structure is

more proper for direct drive use, the PMFSM with outer-rotor has been designed only

for light EV use [14-15]. It provides basically sinusoidal back-electromotive force (emf)

with high torque at low speed. But, constant PM flux of PMFSM makes it hard to

control since it requires field weakening flux at high speed conditions. Moreover, with

quick increase in the price of rare-earth PM on the stator, it will affect the cost of the

motor. While, outer-rotor hybrid excitation flux switching machine (ORHEFSM) with

less rare-earth magnet and field excitation coil on the stator is simple structure of motor

and can give more higher in torque and power density [16].

2.4 Reviews on Flux Switching Motors (FSMs)

In the mid of 1950s the initial idea of flux switching motor (FSM) was founded and

published. Many novel and new FSM topologies have been developed over the last

decade or so for various applications, ranging from low cost domestic purposes,

automotive, wind power, aerospace, and others. The term of flux switching is introduced

due to changing of the polarity of the flux linkage by following the motion of salient

pole rotor.

10

In general, the FSMs can be classified into three groups that are permanent

magnet flux switching machine (PMFSM), field excitation flux switching machine

(FEFSM), and hybrid excitation flux switching machine (HEFSM) as shown in Figure

2.1. PMFSM has only PM, while FEFSM has field excitation coil (FEC) as their main

flux sources. Whereas HEFSM combines both PM and FEC as their main flux sources

[17-18]. Moreover, the FEC can be used to control the generated flux with variable

capabilities. The advantages and disadvantages of FSM must be considered are listed in

Table 2.1 [19].

Table 2.1: Advantages and disadvantages of FSM

Advantages Disadvantages

i. Simple and robust rotor

structure suitable for high

speed applications

ii. Easy to manage magnet

temperature rise as all active

parts are located in the stator

iii. Flux focusing / low cost ferrite

magnets can also be used

iv. Sinusoidal back-emf waveform

which is suitable for brushless

AC operation

i. Reduced copper slot area in

stator

ii. Low over-load capability due

to heavy saturation

iii. Complicated stator

iv. Flux leakage outside stator

v. High magnet volume for

PMFSM

2.4.1 Permanent Magnet Flux Switching Machine (PMFSM)

Permanent magnet flux switching machine have been studied for several decades.

Generally, such machines have a salient pole rotor and the Permanent magnet which is

located in the stator. The salient pole rotor is like to SRMs, which is more robust and

suitable for high speed applications. The difference is only in the number of rotor poles

and stator teeth are two. Difference with conventional IPMSM, the slot area is reduced

when the magnets are moved from the rotor to the stator. To dissipate the heat from the

11

stator it is become easier and the temperature rise in the magnet can be controlled by

proper cooling system.

The general operating principle of the PMFSM is shown in Figure 2.3. The black

arrows show the flux line of PM as an example. When the relative position of the rotor

poles and a particular stator tooth are as in Figure 2.3 (a), the flux-linkage corresponds

to one polarity. But, the polarity of the flux-linkage reverses as the relative position of

the rotor poles and the stator tooth changes as shown in Figure 2.3 (b), the flux linkage

switches polarity as the salient pole rotor rotates.

Figure 2.3: Principle operation of PMFSM

2.4.2 Field Excitation Flux Switching Synchronous Machine (FEFSM)

FEFSM is a form of salient-rotor reluctance machine with a novel topology, combining

the principles of the inductor generator and the SRMs [20-21]. The concept of the

FEFSM involves changing the polarity of the flux linking with the armature winding,

with respect to the rotor position. The viability of this design was demonstrated in

applications requiring high power densities and a good level of durability [22]. The

single-phase FEFSM is very simple motor to manufacture, coupled with a power

electronic controller and it has the potential to be extremely low cost in high volume

applications. Moreover, it inherently offers longer life and very flexible and precise

control of torque, speed, and position at no additional cost by being an electronically

commutated brushless motor,

12

The operating principle of the FEFSM is shown in Figure 2.4. Figure 2.4 (a) and

(b) show the direction of the FEC fluxes into the rotor. Although, Figure 2.4 (c) and (d)

show the direction of FEC fluxes into the stator which produces a complete cycle flux.

As same as PMFSM, the flux linkage of FEC switches its polarity by following the

movement of salient pole rotor which creates the term “flux switching”. The stator flux

switch between the alternate stator teeth for.each reversal of armature current shown by

the transition between Figure 2.4 (a) and (b). The flux will shift clockwise and counter

clockwise with each armature-current reversal.

(a) (b)

(a) (b)

Figure 2.4: Principle operation of FEFSM (a) θe = 0° and (b) θe = 180° flux moves from

stator to rotor (c) θe = 0° and (d) θe = 180° flux moves from rotor to stator

13

2.4.3 Hybrid Excitation Flux Switching Motor (HEFSM)

HEFSMs are those which utilize primary excitation by PMs and secondary source by

DC FEC as their main flux sources has several attractive features. Usually, PMFSMs

can be operated operate beyond a certain speed in the flux weakening region according

to controlling the armature winding current. By applying negative d-axis current, the

PM flux can be prevent but with the weakness of increase in copper loss and thereby

reducing the efficiency, reduced power capability, and also probable irreversible

demagnetization of the PMs. Therefore, HEFSM is another option where the benefits of

both PM machines and DC FEC synchronous machines are joined. The motor with both

PM and FEC located on the stator has the advantage of robust rotor structure. In

addition, as such HEFSMs have the probable to improve flux weakening performance,

power and torque density, variable flux capability, and efficiency which have been

studied comprehensively over many years [23-25].

The operating principle of the proposed HEFSM is shown in Figure 2.5, where

the red and blue line point to the flux from PM and FEC, respectively. In Figure 2.5 (a)

and (b), since the direction of both PM and FEC fluxes are in the same polarity, both

fluxes are combined and move together into the rotor, hence more fluxes known as

hybrid excitation flux will produce. Moreover in Figure 2.5 (c) and (d), the FEC is in

reverse polarity, only flux of PM flows into the rotor while the flux of FEC moves

around the stator outer yoke which less flux excitation.

14

Figure 2.5: The operating principle of the proposed HEFSM (a) θe=0° - more excitation

(b) θe=180° - more excitation (c) θe=0° - less excitation (d) θe=180° - less excitation

2.4.4 Principle Operation of ORHEFSM

In the ORHEFSM, the number of rotor pole and stator slot that possible is expressed by:

(1.1)

where,

Nr = the number of rotor poles

Ns = the number of stator slots k = integer number

q = number of phases

15

In this project, rotor pole number and stator slot number is selected to 10 and 12,

respectively for the three phase motor. In this motor, for the motor rotation through 1/10

of a revolution, has one periodic cycle for the flux linkage of armature. So, the

frequency of back-emf induced in the armature coil is ten times the mechanical

rotational frequency. The relation between the mechanical frequency and the electrical

frequency for the machine can be defined by

(1.2)

where,

fe = the electrical frequency

fm = the mechanical frequency

Nr = the number of rotor poles

The principle operating of the proposed ORHEFSM is shown in Figure 2.6. The

single piece of rotor is shown in the upper part that makes the motor more robust like to

SRM, while the stator that contains of PMs, FECs, and ACs are located in the lower

part. The PM and FEC are placed between two stator poles to generate excitation fluxes

that create the term of “hybrid excitation flux”. Figures 2.6 (a) and (b) show the flux

generated by PM and FEC flow from the stator into the rotor and from the rotor into the

stator, respectively, to produce a complete one flux cycle. The combined flux generated

by PM and FEC established more excitation fluxes to produce higher torque of the

motor. When the rotor moves to the right, the rotor pole goes to the next stator tooth,

hence switching the magnitude and polarities of the flux linkage. The flux does not

rotate but shifts clockwise and counterclockwise in direction with each armature current

reversal. According to Figures 2.6 (c) and (d), only the PM flux flows from the stator

into the rotor and from the rotor into the stator, respectively, while the FEC flux is only

circulating on its particular winding slots. This condition establishes less excitation flux

and generates less torque.

16

Figure 2.6: Principle operation of ORHEFSM (a) θe = 0º (b) θe = 180º more excitation,

(c) θe = 0º (b) θe = 180º less excitation

2.5 Relationship between Torque, Speed and Power

Torque is basically the rotational equivalent of a force. In that sense, torque can be

thought of as the potential to do work. But just as a force can only do work by being

applied through some distance, torque can only do work by being applied through some

angle. The rpm is simply the rate at which that angle changes which is the rate of

rotation.

Furthermore, the speed of an object is the magnitude of its velocity. The average

speed of an object in an interval of time is the distance travelled by the object divided by

the duration of the interval. The instantaneous speed is the limit of the average speed as

the duration of the time interval approaches zero. Like velocity, speed has the

dimensions of a length divided by a time.

While power is the rate at which the torque is doing work. Technically,

Power = Torque * Rotation Rate (1.3)

but usually some constant is included to correct for a convenient choice of units. In

electric vehicle, the vehicle performance is totally determined by the torque-speed

Rotor

Stator

AC

FEC

PM

17

characteristic of the motor which is more close to the ideal as compared to Internal

Combustion Engine (ICE). In order to encounter EV necessity, operation entirely in

constant power is needed. But, operation with fully constant power may impossible for

any practical vehicle. For EVs, the desired output characteristics of electric motor drives

are illustrated in Figure 2.7. It can be observed that the electric motor is expected able to

produce a high torque at low speed for starting and acceleration because as it increase to

the base speed, the voltage increases to its rated voltage while the flux remain constant.

Moreover, electric motor generates a high power at high speed for cruising due to the

fact that beyond the base speed, the voltage remains constant and the flux is weakened.

The result is constant output power while the torque declines hyperbolically with speed.

Thus, a single-gear transmission will be enough for a light-weight EV [26]. The electric

motor can generate constant rated torque up to its base speed. At this speed, the motor

reaches its rated power limit. The operation beyond the base speed up to the maximum

speed is limited to the constant power region. This region of the constant power

operation depends primarily on the particular motor type and its control strategy.

Figure 2.7: Performance Characteristics of Electric Motor

18

From the output characteristics of electric motor for EVs as shown in Figure 2.7, the

following valuable results can be concluded as follows:

a) The rated power for acceleration performance (acceleration time and

acceleration distance) decreases as constant power region ratio increases.

b) On the other hand, the rated torque for acceleration increases as constant power

region ratio increases.

c) The maximum speed of electric motor has a pronounced effect on the required

torque of the motor. Low speed motors with the extended constant power speed

range have a much higher rated shaft torque.

d) As motor power decreases (due to extending the range of constant power

operation), the required torque is increase. Thus, although the converter power

requirement will decrease when increasing the constant power range, the motor

size, volume, and cost will increase.

e) Increasing the maximum speed of the motor can reduce the motor size by

allowing gearing to increase shaft torque. However, the motor maximum speed

cannot be increased indefinitely without incurring more cost and transmission

requirements. Thus, there is multitude of system level conflicts when extending

the constant power range [27].

2.6 The Electric Motor Design Software

Most research efforts to improve motor design have focused on the motor, rather than

the motor-driven system as whole. Simulation tools are useful to address the various

levels of motor and drive system, leading to errors and delays in the design cycle or

increased cost to build and test iterations. There are many types of motor design

software that used to design and analyze electric motor simulation.

The following examples of electric motor design software contain both

analytical tools and finite element analysis (FEA) tools. Emetor from Emetor AB is

online electric motor design software with automated FEA solver. While, MagneForce

Simulation Suite from MagneForce Software Systems Inc. is FEA-based design tool

with modules for synchronous generators, brushless and brush-commutated DC

19

machines, as well as induction machines. State to be the first finite element development

environment exclusively targeted to the design and simulation of electric motor and

generator systems. Although Maxwell from Ansys also FEA tool for electromagnetic

field simulations with optional multiphysics coupling. Maxwell can be enhanced with an

analytical template-based electrical machine design tool. Another software that used by

many electric car company is JMAG from JSOL Corp. JMAG is FEA tool for

electromechanical design, especially adapted for electric motor design [28]. JMAG has

assisted many companies in a wide range of industries, universities and researchers for

the designing. This is because the JMAG software can precisely capture and fast

evaluate difficult physical phenomena inside of machines. Users inexperience and

experienced in simulation analysis can easily perform the simple operations required to

obtain precise results. So in this project, JMAG is also used as a design aid.

CHAPTER 3

METHODOLOGY

3.1 JMAG Designer

In order to reach the objectives of this project, JMAG-Designer version 12.1.02 software

is using to design the motor. JMAG is a comprehensive software suite for

electromechanical equipment design and development. It is also powerful simulation

and analysis technologies give a new standard in performance and quality for product

design. The motor design is dividing into two areas which is design by using Geometry

Editor and then continued using JMAG-Designer as 2D finite element analysis (FEA)

solver to analyse the design. The work flows of JMAG-Designer software are illustrated

in Figure 3.1.

21

(a) Geometry editor flow chart (b) JMAG-Designer flow chart

Figure 3.1: Work flow of project implementation

3.2 Project Implementation in Geometry Editor

Geometry editor is the place to sketch. After click the geometry editor, the toolbar will

appear for the use in designing the parts of ORHEFSM which are the rotor, stator,

permanent magnet, armature coil and field excitation coil. An icons used in the

geometry editor are such as save icon, edit sketch, circle, line, sketch trim, create region,

region mirror copy and region radial pattern. The icon that is used in the process of

designing and description about each icon is depicted in Table 3.1 and Figure 3.2,

respectively.

Start

Rotor Design

Stator Design

Permanent Magnet (PM) Design

Field Excitation Coil (FEC) Design

Armature Coil Design

End

Start

Set the Materials

Set the Conditions

Set the Circuit

Set the Mesh and Study Properties

Run and simulate

Graph

End

22

Table 3.1: Description of icons used in Geometry Editor

Icons

number Description

1 Save model.

2 Edit sketch. It will be used at the beginning to sketch or at the end to edit.

3 Circle. Used to draw a circle.

4 Line. Used to draw a line.

5 Sketch trim. Used to cut the drawing or unwanted lines.

6 Create region.

7 Region mirror copy. Used to make another drawing only by copy it.

8 Region radial pattern. Used to augment existing drawing.

3.2.1 Rotor

Only one part of rotor will be designed. This is because the part that has been designed

can be copy by using region mirror copy. Then, it can form one complete diagram by

using region radial pattern. Both steps in region mirror copy and region radial pattern

can be done if the create region is successful and the drawing need to be merge. To

ensure that create region step is successful, the line should be drawn on each side with

Figure 3.2: Toolbar icons

23

the right scale that mean no intercept point is occurred. Figure 3.3 illustrated the radial

duplication of rotor.

Figure 3.3: Radial duplication of rotor

3.2.2 Stator

In stator designed, the steps are same as rotor design. Only distinguishing is to part of

the stator that needed to draw because their diameter is not the same resulting in

different drawing size and shape. Before step region mirror copy and region radial

pattern done, it must be ensure that the way to create region is success. Region radial

pattern will complete the drawing of the stator. Figure 3.4 represent the radial

duplication of stator.

24

Figure 3.4 Radial duplication of stator

3.2.3 Permanent Magnet (PM)

After drew the stator and rotor part, all the parts that is still blank is filled up. For

permanent magnet part, the steps still the same as before and repeated. One line is drawn

and parallel with another line. Figure 3.5 displays the PM filled in the stator.

Figure 3.5: PM filled in the stator

PM

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